Various methods for electrically connecting bare die integrated circuits to printed circuit substrates are well known in the art. Among the methods of connecting integrated circuits to mated circuit substrates, the technique of flip chip with solder bumps has the most promise with regard to electrical performance, size reduction and price. A flip chip is an integrated circuit chip in which the bonding pads have solder bumps formed on them. Solder bumps are approximately truncated spheres of solidified solder attached to the bonding pads and are typically of a tin-lead composition. The flip chip does not have a plastic shell or metallic leads common to most integrated circuit packages. The active side of the flip chip contains the active devices and bonding pads, and has a passivation layer with apertures, that protects the chip's active components from environmental contaminants. The solder bumps of the flip chip are positioned in registering contact with the substrate circuit conductive contact areas and metallurgically joined to the conductive contact areas during reflow.
Flip chip technology offers a number of advantages over standard wirebonded integrated circuits. Flip chips are generally smaller than wirebonded integrated circuits and require smaller circuit board area. Additionally, flip chips allow for more inputs/outputs (I/Os) than wirebonded integrated circuits.
There are various methods for forming solder bumps. Formation methods include evaporation, electroplating and stencil printing. Each of these methods include limitations. Much research has been performed to overcome the limitations of these methods.
FIG. 1 shows a flip chip substrate 110 in which solder bumps are to be formed on solder-wettable pads 120 of the substrate 110. The solder-wettable pads 120 are formed from an under bumping material deposited over substrate bond pads 140. Typically, the flip chip substrate 110 includes a passivation layer 130. Generally, solder is deposited on the solder-wettable pads 120, and reflowed to form solder bumps on the solder-wettable pads 120.
FIG. 2 shows a flip chip substrate 110 which includes a polymeric mask 210 formed over the flip chip substrate 110. The mask 210 includes apertures 220 which align with the solder-wettable pads 120. Solder paste can be deposited in the apertures 220 of the mask 210. Solder paste is a mixture of solder spheres and a flux vehicle that contains flux and other organic Theological agents. The solder paste is typically 50% solder by volume. The solder paste is then reflowed, forming solder bumps within the apertures 220. The solder bumps are electrically and metallurgically connected to the solder-wettable pads 120.
FIG. 3 illustrates a stencil printing process used to deposit solder paste within the apertures 220 of the mask 210. Here, the mask 210 has been formed on a wafer 330 which includes many flip chip substrates. To deposit the solder paste within the apertures 220 of the mask 210, the mask 210 and the wafer 330 are placed within a printing stencil 310. Some of the solder paste 320 is then "squeegeed" into apertures 220 of the mask 210. The apertures 220 are not shown in FIG. 3. After some of the solder paste 320 has been deposited within the apertures 220 during the squeegee process, the solder paste within the apertures is heated which reflows the solder paste within the apertures forming solder bumps.
FIG. 3 illustrates that due to required alignment tolerances, a gap 340 between the edge of the stencil 310 and the edge of the mask 210 will always exist. During the squeegee process, some of the solder paste 320 will be printed into the gap 340 between the mask 210 and the stencil 310. Excess solder paste 350 must be cleaned from the gap 340 before reflowing the solder paste in the apertures.
If the excess solder paste 350 is not cleaned off, then during reflow of the solder paste in the apertures, the excess solder paste 350 will turn into molten solder balls which can join with the solder bumps being formed causing defects to the flip chip substrate 110 (wafer). In addition, the molten solder balls can form on the side of the wafer 330. The process of cleaning the solder paste is time consuming and adds expense to the solder bump formation process.
It is desirable to have a method of forming solder bumps on a wafer in which a processing step for cleaning excess solder from the wafer is not required.